1. Technical Field
The present invention generally relates to color signal processing. More particularly, the present invention relates to a color signal processing system for a color television receiver which improves a transition characteristic of a color difference signal, using a luminance signal whose transition region has been differentiated.
2. Description of the Prior Art
A conventional color signal processing system, referring to FIG. 1, has a transition detector I for differentiating a transition region of an input color difference signal and for outputting a signal proportional to the slope of the differentiated signal, a high-pass filter 2 for high-pass-filtering the differentiated color difference signal, a comparator 3 for comparing the output from the high-pass filter 2 with a reference signal, a switch 4 for switching the input color difference signal by a signal from the comparator 3, and a capacitor 5 for charging or discharging the color difference signal through the switch 4.
The operation of the conventional color signal processing system will be described with reference to FIG. 2.
If a color difference signal, as a waveform (A) shown in FIG. 2, is provided to the transition detector 1, the transition detector 1 differentiates the color difference signal and outputs a signal proportional to the slope of the transition of the color difference signal, as a waveform (B) in FIG. 2.
The high-frequency component of the differentiated signal from the transition detector 1 is filtered at the high-pass filter 2, and provided to the comparator 3.
The comparator 3 compares, as a waveform (C) in FIG. 2, the filtered signal with in a given reference signal (Ref), and provides a resulting value, as a waveform (D) in FIG. 2, to the switch 4 as a switch control signal.
The switch 4 is opened during the period where the switch control signal remains in a high state, as marked as `tsr` and `tsd` in the waveform (D), while it is closed during the rest of the period.
As a concrete example, during the rapid transition, shown as the waveform (A) In FIG. 2, the switch 4 is opened. At this time a voltage previously charged in the capacitor 5, i.e. a previous value before the transition is output. The switch 4 is closed at the end of the transition, and then the input color difference signal is directly output through the closed switch path.
Whereas during the gradual transition, shown as the waveform (A) in FIG. 2, the switch 4 is opened only at the period of `ts1`, and the value which has been being charged in the capacitor is output.
FIG. 3 shows as an example that the transition time of an input color difference signal is 8 times as long as a luminance signal when providing the waveforms (A) to (C) as a luminance signal, the waveform (D) as an input color difference signal, and a waveform (E) as an output color difference signal processed by a color signal processing system.
Accordingly, if the transition time of a luminance signal and of a color difference signal, which are different from each other, are accorded with each other as waveform (D) in FIG. 3 and if the color difference signal is treated by a color signal processing system, a color difference signal having the same transition time, as waveform (E) in FIG. 3, as a luminance signal is attained.
However, since a color difference signal has been treated without regard to the transition characteristic of a luminance signal in such a conventional color signal processing system, accurate treatment of a color difference signal cannot be achieved at the transition region of a luminance signal on account of the difference of the transition region of a luminance signal from that of a color difference signal.
That is to say, it is not suitable to represent the resolution wherein the frequency band-width of a color difference signal is less than 0.5 MHz (t=0.35/0.5 MHz), because a color difference signal is used for a yardstick for compensation for the transition characteristic.